VAC2022 Building Materials & Construction 5) Properties of hardened concrete (cont.) Introduction - There are certain properties the hardened concrete element attains which can be used in design. - These include stress-strain characteristics, elastic modulus, poisson’s ratio, creep behaviour and shrinkage of concrete. Stress-Strain Characteristics - The presence of aggregate and cement contribute to the stress- strain characteristics of concrete. - Very fine bond cracks (microcracks) exist at coarse aggregate and cement paste interface even prior to the application of load. These are due to differential volume changes between cement paste and aggregate.(ie.stress-strain differences, thermal & moisture changes) - Generally the cracks remain stable and do not grow under stress up to about 30% of ultimate load. Typical stress-strain curves are given below. 40 Agg. Stress Conc. MPa 20 Cement 0 0.001 0.002 0.003 Strain VAC2022 Building Materials & Construction - Concrete curve becomes curvilinear at higher stresses, whereas the aggregate and cement paste curves are linear. This apparent paradox is explained by the development of bond cracks at the interface between the two phases - After 30% of ultimate strength, micro-cracks increase in length, width and number so the strain increases more rapidly than the stress, under sustained load. Together with creep (see later), the development of micro-cracks helps the concrete to redistribute local high stresses to regions of lower stress avoiding early- localised failure. - At 70 to 90% of ultimate strength, cracks open through the mortar matrix and bridge bond cracks so that a continuous crack pattern is formed. This is the fast propagation of cracks and if load is sustained, failure will occur with passage of time. - This is a description of the stress-strain behaviour of concrete in uniaxial compression (using in cubes or cylinders), as indicated by measuring axial strain when the load is increased at a constant rate of stress. - Steeper and straighter ascending curves can be observed for higher concrete strengths. Lower strength concretes are more curved and have a less pronounced initial straight portion. - Loading a specimen at a constant rate of strain, a descending part of the stress-strain curve can be obtained before failure (requires a test machine with stiff frame and displacement is controlled rather than load). Observe the typical curve in the figure below. VAC2022 Building Materials & Construction - Having descending branches in stress-strain curves means concrete can withstand some load after maximum load has been passed since microcrack linking is delayed before complete breakdown. - High Strength concrete and also lightweight concrete implies a more brittle nature than concrete of normal strength. - The area enclosed by the complete stress-strain curve is the work necessary to cause failure or fracture toughness. Deformation Characteristics (Elastic Modulus & Poisson’s ratio) - Elastic Modulus (E) is the most common measure of the elastic property of a material. Ratio of applied stress to the strain. - When subjected to a constant sustained load, the deformation can be divided into two parts: elastic deformation occurring immediately as load applied plastic deformation beginning when load is applied but continues to increase further without load increase. VAC2022 Building Materials & Construction - Concrete is not a truly elastic material and its stress-strain curve has no initial straight part, but the initial straight part is generally used to find the initial elastic modulus. - Other measures include: tangent modulus which is the slope of a tangent to the curve at a point. Has no real use in structural applications. secant modulus which is the slope between the origin and a point on the curve. Easy to find and takes account of total deformation at a point. - E values are not directly related to concrete properties, although higher concrete strengths are accompanied by higher values of E. For normal concretes (20-60 MPa), E lies in the range 18-36 kN/mm2. Also as concrete ages, its E increases. - In AS3600 the modulus of elasticity of concrete is estimated as Ec = ()1.5 x (0.043 fcm) - When subjected in compression, concrete contracts longitudinally and expands laterally. Poisson’s ratio is the ratio of lateral strain to longitudinal (axial) strain. Typical values are 0.15-0.25 - A lateral strain can also be observed when testing in compression. Poisson’s ratio is generally constant for stresses below approx. 30 % of ultimate strength. It increases slowly, and at 70 - 90 % of the ultimate strength, it increases rapidly due to the formation of mainly vertical unstable cracks. VAC2022 Building Materials & Construction Deformation Characteristics (Creep & Shrinkage) - Creep is a non-elastic time-dependent deformation under load, believed to be due to the closure of internal voids and squeezing of water from the cement gel under load. o At present there are several theories proposed to explain creep behaviour. Two major ones are: Seepage theory assumes creep is due to movement (migration) of water from cement gel pores under the action of external applied load. Viscous flow theory assumes creep due to viscous flow of the cement gel itself. - In understanding creep you must recognise the following components of deformation under load: elastic strain is instantly recoverable, the amount of which, may be reduced by creep as E increases with time. recoverable creep Inelastic deformations - The magnitude of creep is therefore as great or greater than elastic strains on loading. Creep is not wholly reversible and some permanent strain remains after creep recovery is complete. VAC2022 Building Materials & Construction - Factors affecting creep include: Aggregates are not liable to creep but their restraining effect on cement paste is. Creep increases as elastic modulus of aggregate decreases. Creep increases as aggregate becomes finer and is greater when porous aggregate is used. Mix proportions. Creep decreases as w/c ratio and volume of cement paste decreases. The movement in a wet mix can be twice as much as in a dry mix. Strength and concrete age. An increase in strength leads to reduction in creep, as stress-strain ratio is reduced. Moisture conditions. Reduced moisture content before loading reduces creep. Temperature increase leads to increase in creep. Curing causes creep to decrease as hydration proceeds, so concrete kept continuously wet creeps less than air cured. Stress Level. Creep increases approximately linearly with applied stress. VAC2022 Building Materials & Construction Member size. Creep decreases with increase in size of specimen. This effect is greatest during the initial period of load application. - Empirical data has been used in several creep prediction methods. As such accuracy is not high and errors of up to 30 % are common. - The CEB methods gives: creep strain (c) = ( x )/ Ect is applied stress, is creep coefficient, Ect is elastic modulus of the concrete at the age of loading and is calculated from Ect = Ec28(0.4 + 0.6fct/fc28) Ec28= 20 + 0.2 fc28 = Kb x Kc x Kd x Ke x Kt Kb = effect of mixing proportions, Kc = effect of drying under load, Kd = effect of ageing and hydration, Ke effect of member size, Kt = effect of time. This method has been criticised for having too many factors. A graph of has then been developed knowing relative humidity (exposure conditions) and effective section thickness (2 x cross section area divided by exposed parameter). - Shrinkage of concrete is caused by settlement of solids and loss of free water from the plastic concrete (plastic shrinkage), by the chemical combination of cement with water (autogenous shrinkage (swelling)) and by the drying of concrete (drying shrinkage). VAC2022 Building Materials & Construction Where movement of the concrete is restrained, shrinkage will produce tensile stresses within the concrete causing cracking. The components of shrinkage are: Plastic shrinkage: an inital reduction in paste volume occurs due to water absorption by cement particles and evaporation of water from the surface. Autogenous shrinkage takes place in hardened paste and is the result of the hydration process (hydrated gel has a smaller volume than the sum of cement and water). Prominent in sealed specimens or interior of large concrete mass. Small compared to plastic shrinkage. Autogenous swelling occurs if concrete is kept saturated during curing permitting water to be absorbed by the gel particles giving rise to swelling. Drying shrinkage occurs as the volume of hardened concrete decreases by an amount proportional to gel water loss. That is, the volume contraction occurring as the concrete hardens and dries out. It is high compared to the other shrinkage types. VAC2022 Building Materials & Construction - Factors affecting drying shrinkage include: Aggregates Addition of aggregate reduces the amount of dimensionally unstable component (cement paste) and produces a restraining effect to prevent volume change. Specimen geometry. The greater the surface area per unit mass, the greater the rate of shrinkage. Cement Type. Water and cement contents. Generally drying shrinkage is directly proportional to the w/c ratio. Also with a given w/c ratio shrinkage increases with increasing cement content. Curing Longer periods of curing can give rise to increased shrinkage for thin sectioned specimens. Relation between tensile and compressive strengths - The theoretical compressive strength can be up to 10 times larger than the tensile strength. The ratio of tensile to compressive strength is lower the higher the compressive strength. - Factors affecting the relation between the two strengths: VAC2022 Building Materials & Construction # the method of testing the concrete in tension, # the size of the specimen, # the shape and surface texture of coarse aggregate and # the moisture content of the concrete. The figure below shows typical values of tensile strength as a function of compressive strength for the different methods of testing. The flexure test is on 100x100x500 prisms, splitting test on 150x300 cylinders, direct test on 75x355mm bobbins and compression test on 100mm cubes. The Figure shows moisture condition influences results. 8 6 Tensile strength flexure (wet) MPa 4 flexure (dry) splitting (wet & dry) 2 direct (wet & dry) 0 10 20 30 40 50 60 70 80 90 100 Compressive Strength - MPa - A number of empirical formulae have been suggested to relate tensile (ft) to compressive (f’c) strengths. Most are of the type: ft = k f n c where k and n are coefficients depending on factors such as specimen shape and properties of the mix used. The expression used in ACI code is : ft = k fc In AS3600 the principal tensile strength is: fct = 0.4 f’c In AS3600 the flexural tensile strength is: fct = 0.6 f’c VAC2022 Building Materials & Construction Non-destructive testing - Non-destructive testing is particularly useful for assessing the quality of concrete in the finished structure itself. In short, the element under test will not be damaged. - The typical non-destructive tests are: ultrasonic pulse method: # measures the velocity of ultrasonic pulses (3 to 5 km/s) passing through the concrete from a transmitting transducer to a receiving transducer. -The higher the velocity, the greater the strength of concrete. # The ultrasonic velocity depends on the elastic modulus of concrete and since it is related to compressive strength, the pulse can be correlated to strength. The correlation also depends on mix proportions and aggregate type. - Thicknesses of 100 to 5000 mm are tested satisfactorily. (Schmidt hammer) Rebound Hardness Test # It consists of a metal plunger, one end of which is held against the concrete surface with the free end struck by a spring loaded mass which rebounds, the magnitude of which measures the hardness of the surface. VAC2022 Building Materials & Construction # A rebound number results. Hardness is a relative property and there is no physical relationship between it and other properties of concrete. Empirical relationships between rebound number and strength have been established. The higher the number, the greater the strength. # Although popular because of its simplicity, it only provides an approximate indication of quality since it tests only the quality of concrete near the surface (about 30 mm deep). Other tests # the use of Gamma rays to detect voids in concrete and locate reinforcement. # electrical methods to measure moisture content. # electromagnetic methods to measure the depth below the concrete surface of steel bars. VAC2022 Building Materials & Construction 6) Concrete Production (Ch 6-9) Introduction This section deals with material handling and production practices which is essential in ensuring that the finished concrete achieving the specification required. Material Batching AS 1379- Specification and Manufacture of Concrete: Aggregate: Stockpiles of different aggregate sizes formed some distance apart. If close together provide some from of partition. Batching by weight (rather than measuring by volume) eliminates errors due to variations in the proportions of voids contained in a specified volume. Weighing machines on site need careful maintenance and regular calibration if reasonable accuracy is to be maintained. On smaller sites, material is weigh-batched in a loading hopper. The hopper has a pressure gauge giving a direct progressive reading of the total weight of the material. When the hopper has the required batch weights of each material, it is raised allowing contents to fall through a discharge point directly into the mixer. On mixing plants, the aggregate is fed from an overhead hopper on a short conveyor belt at a controlled rate; with continuous weighing. VAC2022 Building Materials & Construction Cement: batched either by bag (40 kg), or by bulk from a silo installed on site. If bags are used, adjust your batch size to make use of the full bag whenever possible. Make sure that cement flows freely and continuously from the storage hopper. Advantage of bulk cement: # cheaper than bagged cement # less man-power # less trouble in storing # less errors due to bag splitting # changes in mix proportions are made easily. Water: The first few batches of concrete should be produced under supervision to ensure workability requirement are met. People tend to add more rather than less water to the mix. Water measurement devices are generally one of two main types: vertical tanks (equipped with a calibrated gauge) or water meters (fitted into the main water supply line to the mixer). Admixture: usually dispensed in liquid form in amounts which is very small in relation to the batch size. The most common method is to inject the admixture into the water line to the mixer. The timing of introducing the admixture is important to ensure uniformity. Liquid- dispensing equipment can be used and should be equipped with a visual metering device. VAC2022 Building Materials & Construction Mixing Types of machine available include revolving or stationary pan, with paddles to mix the materials, others having tilting revolving drums. Inclined-drum mixers mounted on trucks (transit mixers) are the most common in the industry. Receive accurately batched materials from a central batching plant and, operating at mixing speed, mix the concrete en-route to the site. By reversing the direction of rotation of the drum, concrete discharges continuously from the drum. A split-drum mixer is normally of 2 m3 capacity. The drum which rotates on a horizontal axis, separates into two halves along a vertical plane, allowing the concrete to be discharged rapidly. The two halves are closed during charging and mixing. AS 1379 lmits the capacity of the mixer to no more than 65% of its gross internal volume to ensure efficient mixing. Loading above the rated capacity will increase mixing time or result in incomplete mixing or even spillage of materials. Too short a mixing time will result in a patchy non-uniform and low strength concrete. Site mixing is not so common, however, you need to know the following principles: # Set up the mixer correctly with its axis of rotation drum is horizontal. Inaccurate setting may result in poor mixing. # The order of feeding the ingredients into the mixer is not important but using the right quantities is. # Mixing should continue until the concrete is of uniform consistency and colour. # Run the mixer at the correct speed and do not overload. # keep a high level of supervision. VAC2022 Building Materials & Construction # Regular cleaning of blades is required to ensure efficiency. # Avoid excess of water, otherwise you will have: lower comp. strength, segregation, and higher tendency toward shrinking leading to cracking. Transporting To be rapid to avoid premature setting, segregation and to keep the required level of workability which may be affected by: Ambient temperature, humidity, type of cement, absorbency of aggregate. In the case of ready-mixed concrete, the usual specified requirement is that the concrete shall be discharged from a truck mixer within one hour after the time of loading. An advantage is to keeping the concrete agitated by slow rotation of the drum as it permits longer intervals of time between initial mixing and placing of concrete. Planing a good access to the site is important factor in avoiding delays and interruptions. The delivery rate which can be achieved on a site may be determined by the access to the site, or the rate at which concrete is handled and placed. Various methods are available for transporting concrete; wheelbarrows, dumpers, crane buckets, conveyors, concrete pumps with pipe lines. VAC2022 Building Materials & Construction Placing Ensure that there is no segregation of the mix and a thorough compaction is achieved. Careless placing may cause movement of steel bars, or damage of formwork. Precautions to be taken during placing include: # Concrete should be deposited as near as practicable to its final position and should not be deposited in a large quantity. # Concrete should be deposited in horizontal layers. As far as practical, each layer should be placed in one continuous operation (200-300 mm are common layer thickness) # Concreting should be carried out continuously in order to avoid the appearance of an unsightly lift line on the finished structure. # Concreting should be worked thoroughly into position around the reinforcement and into the corners of the formwork. Provided there is no obstruction from steel bars, the most satisfactory placing method is by free discharge from a container moved slowly along the line of the member under construction. To ensure maximum density of the plastic concrete some form of vibration is required. Compaction ensures an intimate contact between concrete and the surface of reinforcing bars. The amount of vibration required to achieve compaction will be dependent upon many factors including the mix design, maximum aggregate size, slump value, and the amount of steel. In addition to expelling entrapped air, compaction promotes a more even distribution of pores within the concrete, causing them to be discontinuous, ie. permeability is reduced. Also vibration reduces the VAC2022 Building Materials & Construction internal friction between the aggregate particles so that they pack together more tightly and permit entrapped air to rise to the surface. Excess of vibration can cause segregation as larger sizes will tend to sink to the bottom and a layer of mortar will form at the surface. Main types of compaction equipment are vibrators & vibrating-beams Common sizes of internal vibrators (pokers) are 25 and 150 mm diameter. Consist of a flexible tube containing vibrating units and is generally powered by a petrol engine compressed air or electric motor. Typical operating speed is 10,000 rpm. The larger the aggregate, the lower the workability and the greater is the compaction effort. External vibrators are usually rigid attached to the forms by means of clamping devices and cause a shaking motion of the form through which the pulsations are distributed to the concrete. Form vibrations are adopted when it is impossible to use internal vibrators. A surface vibrator (vibrating-beam screed) consists of one or two metal beams on which is mounted an electric vibrating unit similar to the external vibrator. These surface vibrator are used for pavement construction, or in combination with internal ones, for compacting concrete in large structures such as dams and retaining walls. VAC2022 Building Materials & Construction Finishing - This process involves leveling (screeding), floating and troweling. Screeding concrete is a leveling operation that removes humps and hollows and gives a true, even surface to the concrete. It should be done before bleeding water rises to the surface. The surface is struck off by moving a straight beam back and forth across the surface. A small amount of concrete is always kept ahead of the beam to fill in the low spots and maintain a plane surface. Floating is to impart to the concrete surface a relatively even but still open texture in preparation to other finishing operations, and thus to: Embed large aggregate particles beneath the surface Remove slight imperfections & produce true plane surface Consolidate the surface mortar in preparation for other finishing operations. Close minor surface cracks that appear as surface dries. Note: Floating should not begin until all bleed water has evaporated from the surface and the concrete has begun to harden, where it can withstand the finisher walking on it with only minimum indentations to the surface. Floating can be achieved by either hand (hand floats) or power troweling machine equipped with flat floats (shoes). Troweling is carried when a smooth, dense surface is desired, usually some time after floating. The delay is to allow some stiffening to take place but too long a delay results in a surface that is too hard to finish. VAC2022 Building Materials & Construction First troweling, the blades should be kept as flat against the surface as possible since tilting or pitching the trowel can create waves (ripples). Should produce a smooth surface free of defects. Additional troweling may be used to increase the smoothness and wear resistance of the surface. Successive trowelling, with a break to allow further hardening, will densify the surface, providing increased wear resistance. Successive trowelling should be at right angles to each other for maximum effectiveness.